1. Field of the Invention
The present invention relates to a grain oriented electrical steel sheet used for cores of transformers and generators, specifically to a production process for a grain oriented electrical steel sheet having an ultralow iron loss.
2. Description of the Related Art
Grain oriented electrical steel sheets which contain Si, and have crystal grains strongly oriented to the (110) [001] orientation and the (100) [001] orientation, have excellent soft magnetic characteristics. This allows the grain oriented electrical steel sheets to be widely used as core materials for transformers and for generators used in the commercial frequency band.
Among characteristics required in such uses as core materials, it is important that the iron loss expressed as W.sub.17/50 (W/kg) be low. Such loss is generally the loss observed when magnetization to 1.7 T is achieved in a frequency of 50 Hz. Electric power loss in transformers and generators can be substantially reduced by using materials having low W.sub.17/50 values. Accordingly, grain oriented electrical steel sheets having improved low iron loss have been strongly required year by year.
In general, in order to reduce iron loss of a grain oriented electrical steel sheet, several methods are available. One is a method in which the Si content is increased, another in which thickness of the steel sheet is reduced, another in which crystal grain diameter is reduced, and still another in which the alignment degree of the crystal grain orientation is increased.
The electric resistances are also elevated in these methods, and therefore the eddy current loss out of the iron losses is lowered. The magnetic flux density is enhanced in the methods, and therefore the hysteresis losses out of the iron losses are reduced.
However, excess addition of Si deteriorates the rolling workability and processability, and therefore is limitative and not preferred. The method is further limitative since it requires extreme increase of production cost. Also, since excessive reduction of crystal grain diameter lowers the alignment degree of the crystal grain orientations and increases hysteresis loss, the iron loss undesirably grows larger.
The subject has so far been thoroughly investigated.
Disclosed in, for example, Japanese Examined Patent Publication No. 46-23820 is a technique in which Al is added to a steel and fine AlN is precipitated by hot rolled sheet annealing at high temperatures of 1000 to 1200.degree. C. after hot rolling and quenching treatment following with cold rolling at a high rolling reduction of 80 to 95%. A very high magnetic flux density of 1.95 T in B.sub.10 (magnetic flux density in a magnetic field of 1000 A/m) is obtained by this method. According to this method, AlN finely dispersed and precipitated has a strong action as an inhibitor controlling the growth of primary recrystallized grains. Only nuclei having excellent grain orientations are secondarily recrystallized by strong inhibiting effect to provide products having a crystal grain structure with excellent orientations. In this method, however, crystal grains are usually coarsened, and the eddy current loss grows larger. Accordingly, it is difficult to obtain a low iron loss. Further, it is difficult to make AlN completely solid solute in hot rolled sheet annealing, and therefore it has been difficult to stably obtain products having a high magnetic flux density.
Further, disclosed in Japanese Unexamined Patent Publication No. 2-115319 is a method in which Sb is further added to steel as a segregation type inhibitor to carry out a specific final annealing method. A product having a high magnetic flux density was obtained by this method, but the grain orientation alignment degree was not satisfactory. When the Sb content was increased in order to obtain a product having a higher alignment degree, the secondary recrystallization became unsatisfactory, and the iron loss was degraded to a large extent.
Further, disclosed in Japanese Examined Patent Publication No. 58-43445 is a method in which a steel containing 0.0006 to 0.0080% of B and 0.0100% or less of N is used to devise decarburization annealing. A magnetic flux density of 1.89 T in B.sub.8 (magnetic flux density in a magnetic field of 800 A/m) was obtained by this method. This method provides products having relatively stable magnetic characteristics and therefore is preferred from a practical point of view. However, this method has not become industrial because the magnetic flux density is low and the iron loss is not good.
Further, disclosed in Japanese Examined Patent Publication No. 54-32412 is a technique in which S or Se is used in combination with a member of the group of As, Bi, Pb, P, Sn, Cu or Ni. The high magnetic flux density was relatively stably obtained by this method, but the iron loss was not good.
Separately from these techniques, disclosed in Japanese Unexamined Patent Publication No. 2-30718 is a method in which grooves are provided on the surface of a product sheet by forming grooves on the surface of a steel sheet after cold rolling, and the eddy current loss is reduced to lower the iron loss. According to this method, however, the magnetic flux density is reduced and the hysteresis loss grows larger, and therefore a large iron loss reduction effect is not obtained.
Further, disclosed in Japanese Unexamined Patent Publication No. 5-345921 is a technique in which a prescribed amount of Ni is provided according to the ratio of Si content to C content in a grain oriented electrical steel sheet containing AlN, MnS and Cu and Sn as inhibitors. However, the product did not have a satisfactory grain orientation alignment degree, and the iron loss was not good.
As described above, the alignment degree of grain orientation has to be increased stably in order to reduce the iron loss of a grain oriented electrical steel sheet. Higher alignment of grain orientation makes it possible stably to obtain an excellent iron loss value.
An object of the present invention is to provide a technique for highly aligned grain orientations.
In conventional techniques, the crystal grain diameter inevitably increases when the alignment degree of grain orientations is raised. As a result, eddy current loss is increased, and iron loss value is degraded in a certain case. Accordingly, such techniques are unstable in terms of production conditions.
In contrast with this, the alignment degree of grain orientation is inevitably lowered when crystal grains are attempted to be refined. As a result, magnetic flux density is reduced, hysteresis loss grows largerr and iron loss value is reduced in some cases. Accordingly, such technique is unstable as well in terms of production conditions.
That is, in conventional techniques, refining of crystal grains could not be compatible with high alignment of grain orientations. Accordingly, materials having a very high magnetic flux density and a low iron loss could not stably be produced.
Another object of the present invention is to cause the conditions of a crystal grain, which have so far been inconsistent, to stand together and to resolve them radically. That is, in a production process for a grain oriented electrical steel sheet using AlN as an inhibitor, an object of the present invention is to provide a technique for obtaining a very high B.sub.8 value and solving the instability of coarsening of crystal grain diameter of the product.